The invention relates to a wireless network for a metropolitan area. More particularly, the invention relates to a technique for maintaining a predefined transmission link quality in a wireless network for a metropolitan area.
Computers utilized in modem office environments are typically coupled to a local area network (LAN). The LAN allow users of the computers to share common resources, such as a common printer included in the network, and allows the users to share information files, such as by including one or more file servers in the network. In addition, the users are typically able to communicate information with each other through electronic messaging. A commonly utilized type of LAN is Ethernet. Currently, a variety of products which support Ethernet are commercially available from a variety of sources.
Business organizations and their affiliates are often spread over several sites in a metropolitan or geographical area. For example, a business organization can have a headquarters, one or more branch offices, and various other facilities. For such business organizations, LANs located at the various sites will generally need to communicate information with each other. Wireless communication links for connecting local area networks are known. For example, U.S. Pat. No. 4,876,742, entitled xe2x80x9cApparatus and Method for Providing a Wireless Link Between Two Area Network Systems,xe2x80x9d and U.S. Pat. No. 5,436,902, entitled xe2x80x9cEthernet Extender,xe2x80x9d each disclose a wireless communication link for connecting LANs.
Availability of a wireless link is commonly expressed as the percentage of the time during which the bit error rate representing the performance of the link is lower than a given threshold level. Environmental precipitation causes a severe attenuation of the transmitted signal, especially for links operating in millimeter frequency bands. For example, to maintain an availability of 99.99% in the presence of environmental precipitation, the signal must be transmitted at a level as high as 24 dB/km higher than otherwise. Therefore, to ensure an acceptable data error rate under all expected conditions, data is typically communicated over a wireless communication link at a relatively high power and at a relatively low rate. The amount of data required to be communicated over the wireless link, however, can vary widely over time and can vary independently of environmental conditions. In addition, wireless links, especially those operated at high power levels, can cause interference with other wireless links operating in the same geographical area.
Therefore, what is needed is a technique for maintaining a predefined transmission quality while transmitting data over a wireless communication link in a metropolitan area network (MAN). What is further needed is a technique for transmitting data over a wireless communication link in a MAN at a rate sufficient to keep up with demand. What is still further needed is a technique for transmitting data over a wireless communication link in a MAN at a relatively low power. What is additionally needed is a technique for reducing interference between wireless communication links operating in the same geographical area.
The invention is a method and apparatus for maintaining a predefined transmission quality for transmitting data in a wireless metropolitan area network (MAN). Each of a plurality of local area networks (LANs) are coupled to a corresponding router. Each router is coupled to one or more transceivers for interconnecting the routers via wireless communication links, thus forming the MAN. Data for communication over the MAN is generated by a user or application at a source node in a source LAN and is communicated to a source router coupled to the source LAN. The data is used to modulate a carrier signal. A transceiver for the source router transmits the modulated signal along a path of one or more wireless communication links to a receiver of a destination router for the data. If the path includes more than one wireless link, one or more intermediate routers will receive and retransmit the modulated signal along the path. The destination router demodulates the signal and communicates the data to a destination node within a destination LAN.
Each router is coupled to one or more of the wireless links over which the router can selectively transmit data. In addition, each router stores a table representative of the topology of the entire MAN. Each router receives detected indicia representative of the transmission quality of wireless links in the MAN and detected indicia representative of network demand. Based upon the stored topology and detected indicia, each router determines a manner in which data is to be transmitted over a selected one of the corresponding wireless links.
In contrast to conventional wireless communications, the present invention maintains an overall availability of 99.99% or better by adapting the manner of transmission and the path of transmission to environmental and other conditions (by detecting transmission quality) and to demand placed upon the network (by detecting traffic loads) and by reliance upon the delay tolerant nature of packet switched communications. As a result, a MAN can be constructed according to the present invention that is highly efficient, in terms of cost, power and bandwidth utilization in order to achieve larger coverage and higher network density.
An example is provided of adapting the manner of transmission to detected transmission quality. The strength of the signal received over each wireless communication link is periodically monitored to determine whether the received signal strength is within a predefined range. If the signal strength falls outside the range, the power at which the modulated signal is transmitted is adjusted to return the signal strength to the range. In addition, the error rate for data received over each link is monitored.
If the received signal strength for a communication link remains below the predefined range despite the transmit power having been adjusted to a maximum allowable level, and if the error rate for data received over the wireless link approaches or exceeds a predefined limit, one or more techniques are employed to reduce the error rate while maintaining a sufficiently high network throughput. A first technique for reducing the error rate is to reduce a rate at which data is communicated along a path. A second technique for reducing the error rate is to alter a modulation level of transmitted signal along the path. A third technique for reducing the error rate is to alter an error correction coding scheme employed for data communicated along the path. A fourth technique is to utilize spectrum spreading for communicating the data along the path.
Each of these techniques for reducing the error rate for data communicated along a communication path is dynamically utilized, singly or in combination, for maintaining an error rate for the data below the predefined limit while maintaining sufficient data communication bandwidth to keep up with demand placed upon the MAN. By transmitting data over each link at a relatively low power, and by utilizing error correction coding and spectrum spreading, interference between links is kept to a minimum, allowing for a high network density in the MAN.
In addition, the amount of data communicated over each link is also monitored. The communication path is preferably selected according to an Open-Shortest-Path-First (OSPF) algorithm. If the shortest path does not have sufficient available bandwidth or results in an excessive error rate, a technique for reducing the error rate and for increasing the amount of data communicated is to select one or more alternate paths for communicating at least a portion of the data. Preferably, the alternate paths are selected to be the next shortest available paths.